A transmission mounted on a vehicle has, as a conventional oil pump used for transferring oil (lubricating oil or hydraulic fluid) for lubricating and cooling a gear and a rotation shaft, a mechanical oil pump (gear pump) driven by a rotation transmitted from a driving source of a vehicle such as an engine. In a design of this kind of oil pump, a flow rate of oil with safety factor is set from past records and experience values so as to secure a required flow rate in a rotation range lower than a specified rotational speed. Therefore, in a high rotation range exceeding the specified rotational speed, the oil pump pumps up a flow rate of oil higher than required. Therefore, in such mechanical oil pump, redundant driving torque (pump driving force) occurs in the high rotation range exceeding the specified rotational speed. And such driving torque has been a loss to the driving source such as the engine. In this regard, as a mechanism for reducing the driving torque of the mechanical oil pump, a mechanism for changing a flow rate and a mechanism for reducing a pump rotational speed are commonly applied. As the mechanism for changing the flow rate, for example, Patent document 1 proposes a mechanism having a combination of a hydraulic pressure sensitive spool valve and a relief circuit (a variable flow rate pump with a relief circuit). Further, as the mechanism for controlling the pump rotational speed, for example, Patent document 2 proposes the mechanism for reducing the pump rotational speed via a planetary gear.
However, the above-mentioned variable flow rate pump with the relief circuit has many component parts, requiring a high degree of dimensional accuracy. This might complicate a structure and leads to an increase in weight and cost. In addition, such complicated structure entails a problem of resistance to micro foreign substances (contamination) such as metal powders contained in the oil. Further, the mechanism for reducing an input rotational speed of the pump using the planetary gear has a problem that a complicated structure of the mechanism may upsize such mechanism having the planetary gear.